Improper electrophoresis conditions were used. Do not allow voltage
to exceed ~20 V/cm. Maintain a temperature <30°
C during electrophoresis. Check that the electrophoresis buffer
used had sufficient buffer capacity. This is done by checking the pH in
the anode and cathode chambers.

There was too much salt in the DNA. Use ethanol precipitation to remove
excess salts, prior to electrophoresis.

The DNA was contaminated with protein. Use phenol extractions to remove
protein prior to electrophoresis.

Small DNA bands diffused during staining. Add the
ethidium bromide during electrophoresis.

If you see anomalies DNA band migration:

Improper electrophoresis conditions were used. Do not allow voltage
to exceed ~20 V/cm. Maintain a temperature <30°
C during electrophoresis. Check that the electrophoresis buffer
used had sufficient buffer capacity.